1. Field of the Invention
The present invention relates to a motor vehicle control, and more particularly to an automatic cruise speed control of a motor vehicle. More specifically, the present invention pertains to an automatic constant cruise speed control of a motor vehicle.
2. Description of the Prior Art
In recent years, there are motor vehicles which are provided with automatic speed control systems with which the vehicle speeds are automatically controlled at desired speeds without necessitating actuations of the vehicle accelerator pedals. In these motor vehicles, the operator is required only to adjust the vehicle speed at a desired value and press the control start switch. The control unit memorizes the vehicle speed at which the control switch is actuated and automatically controls an engine output control member such as the engine throttle valve or the fuel control unit.
In a motor vehicle having the aforementioned automatic control system, when the vehicle comes to an uphill slope, the control unit functions to open the engine throttle valve or to increase the fuel supply to the engine so that the desired vehicle speed is maintained. It should however be noted that when the engine throttle valve is fully open but the vehicle speed still continues to decrease it is no longer possible to maintain the desired vehicle speed. Thus, it will become necessary to shift down the vehicle transmission. In a vehicle having an automatic transmission, such shift down may be conducted by fully depressing the accelerator pedal to close the kick-down switch. Thus, the operator will be required to make an additional operation in order to maintain a desired speed in an uphill slope.
In view of the above inconveniencies, Japanese patent disclosure No. 58-192114 proposes a control system wherein the vehicle transmission is automatically shifted down when the vehicle speed is decreased. According to the proposal, control circuits are provided for judging that the actual vehicle speed is decreased below a reference speed by a predetermined value and that the engine throttle valve is in the wide open position and shifting the transmission down to a lower gear stage. In the proposed system, it will be possible to recover the vehicle speed to the desired value even when there is a certain drop in the uphill drive. It should however be noted that there is a high possibility that the transmission is frequently shift down and up in an uphill operation producing a gear shift shock in each gear shift operation.
In case where the vehicle provided with an automatic transmission having an overdrive gear stage wherein the gear ratio is smaller than 1.00, it becomes difficult to obtain an engine brake effect in an operation on a steep downhill slope with the overdrive gear stage. Thus, the vehicle speed may increase beyond the desired value even when the engine throttle valve is fully closed. In order to solve the problem, the aforementioned Japanese patent disclosure proposes to shift down the transmission to the direct drive gear stage. With this control, it may become possible to maintain the vehicle speed substantially at the desired value. It should however be noted that, even in this control, the transmission will be shifted down and up very frequently possibly producing unpleasant gear shift shocks.
In the Japanese patent disclosure No. 57-196317, there is disclosed a vehicle speed control in which the transmission gear stage is shifted down when the vehicle speed is decreased below a desired value by a first predetermined value, for example 7 km/h, and shifted up when a predetermined delay time has passed after the vehicle speed is increased to a second predetermined value, for example, 3 km/h which is higher than the first predetermined value. By providing the delay time, the control system may be effective to a certain extent to suppress the frequent shifting operations. It should however be noted that the proposed system is not satisfactory because two reference speed values are provided and therefore the first predetermined value must be deviated from the desired speed by an amount which is not so small. Thus, there is a possibility that the vehicle speed may be decreased to a value slightly higher than the first predetermined value and maintained at the value.